The ACIS (.sat, .sab) file format is a geometric modeling kernel developed by Spatial Corporation (now part of Dassault Systèmes). It's a widely used format for representing 3D solid models in CAD (Computer-Aided Design), CAM (Computer-Aided Manufacturing), and CAE (Computer-Aided Engineering) applications. SAT (ACIS Text) files are ASCII-based and human-readable, allowing for easier inspection and debugging, while SAB (ACIS Binary) files are binary-based, resulting in smaller file sizes and faster read/write operations. ACIS files store precise geometric information, including curves, surfaces, and solid bodies, along with topological relationships between these entities. This allows for accurate representation of complex 3D shapes. The format supports a wide range of geometric primitives, such as lines, arcs, splines, surfaces, and solids. It's used for exchanging 3D model data between different CAD systems and for storing models for analysis and manufacturing purposes. The ACIS kernel is embedded in many commercial CAD packages, making .sat and .sab files a common interchange format.